The present invention pertains to the field of satellite communications, in particular, frame synchronization techniques in Time Division Multiple Access (TDMA) based systems for communication between multiple remotes and a hub over a satellite link.
A geostationary satellite is an earth orbiting satellite that generally appears to be stationary relative to any point on earth. As will be discussed at great detail later, geostationary satellites “drift” somewhat relative to a point on the Earth. However, generally speaking, they remain stationary. Some of the applications of the geostationary satellites include telephone, internet, television, radio and communication services. Numerous remote sites (remotes) communicate with a central hub through the satellite using TDMA channels. TDMA allows multiple channels to transmit intermittently on the same frequency, but with the timing of their transmission so arranged that the bursts do not overlay when they arrive at the satellite but arrive in sequence and thus are all successfully received by the hub.
In a typical communication system, a hub serves a plurality of remotes located in a set of spot beams. This is further explained with the help of a FIG. 1.
FIG. 1A illustrates an example layout of four spot beams served by a hub.
As illustrated in the figure, a United States map 102 includes the geographical placement of a beam 104, a beam 106, a beam 108, a beam 110 and a hub 112.
Hub 112 serves a plurality of remotes located in beam 104, beam 106, beam 108 and beam 110 through a satellite (not shown). Hub 112 and its remotes are connected in a star topology with hub 112 at the center of the star and the remotes at the point of the star. In this example only one hub is shown, however, there may be multiple hubs, each serving its own set of beams. In another example, a hub located in one continent may be serving the beams located in other continents. Location of remotes within a beam is further explained with the help of a FIG. 1B.
FIG. 1B illustrates beam 104 with a set of remotes.
As illustrated in the figure, a remote 114, a remote 116 and a remote 118 are located inside beam 104, whereas a remote 120 and a remote 122 are located outside beam 104. In this example, only remote 114, remote 116 and remote 118 are served by hub 112 since remote 120 and remote 122 are located outside beam 104. Remote 120 and remote 122 may be served by another hub.
FIG. 2 illustrates communication between a hub and the remotes via a satellite.
As illustrated in the figure, hub 112 communicates with remote 114, remote 116 and remote 118 via a satellite 202. Hub 112 and its remotes are connected in a star topology with hub 112 at the center of the star and the remotes at the point of the star. Remote 114, remote 116 and remote 118 use TDMA to access shared inroute channels for transmissions through satellite 202 to hub 112. Several remotes share one inroute channel to communicate with hub 112, hence sharing the bandwidth. There could be several inroutes associated with one outroute. As illustrated in the figure, remote 114, remote 116 and remote 118 are allocated a frequency f1 and time slots t1, t2 and t3 respectively in TDMA mode.
For a given beam, the propagation time from a remote to the hub through the satellite can vary by several milliseconds (ms) for different remotes within a beam. For example, for beam 104, the propagation time for remote 114 is d1, for remote 116 is d2 and for remote 118 is d3, where d1, d2 and d3 may have same or different values. Further, as satellite 202 moves in its orbit, the propagation time between each of remote 114, remote 116 and remote 118 and hub 112 via satellite 202 varies continuously. The change in the propagation delay as a result of the change of position of the satellite is a drift delay.
TDMA requires that each remote transmits its data bursts to the satellite for relay to the hub such that the bursts start within a narrow window of time, known as the aperture, in a specified slot of a particular frame at the hub. The variations in the propagation delay between the remotes and the hub require that each hub and remote execute procedures to determine exactly when the remote should transmit a data burst, so that it will arrive at the hub in the assigned frames at the assigned times (i.e., within the aperture). This is explained below for a TDMA frame with the help of FIG. 3.
FIG. 3 illustrates a typical TDMA frame.
As illustrated in the figure, a TDMA frame 300 includes a time axis 302, a time axis 304, a slot 306, a slot 308 and a slot 310. In this example, slot 306 is assigned to remote 114, slot 308 is assigned to remote 116 and slot 310 is assigned to remote 118. A guard period 312 is assigned between each slot to allow for slight mistiming between each burst in order to avoid collision at the receiver. In most cases, a unique word (UW) is transmitted at the beginning of each burst for the receiver to recognize the start of a burst (not shown).
Hub 112 broadcasts each remote a burst time plan in order to assign particular time slots to use in TDMA frame 300. The burst time plan may be fixed, so as to assign each remote a particular portion of the total TDMA frame period or it may be dynamic, whereby the timeslot allocated is adjusted in response to the traffic needs of each remote.
Depending on the geographical location of the remote with respect to the hub, each remote should transmit its bursts such that all the bursts arriving at the hub should appear in the correct place. Remotes further away from the satellite should transmit earlier than the remotes closer to the satellite. Referring back to FIG. 2, remote 114 is closer to hub 112 than remote 118 (d3>d1), therefore remote 118 should start transmitting before remote 114. Similarly, remote 116 is closer than remote 114 (d1>d2), therefore remote 114 should start transmitting before remote 116. This would guarantee that the bursts from remote 114, remote 116 and remote 118 will arrive at hub 112 in the correct sequence at the assigned times.
As discussed with reference to FIG. 3, an inroute TDMA frame 300 consisting of bursts from remote 114, remote 116 and remote 118 arrives at hub 112 in the correct sequence after a certain delay corresponding to the propagation delay between the remotes and the hub.
Hence multiple remotes can communicate to the hub via a satellite using the same inroute frequency in a TDMA based system so that there are no collisions. When the satellite is stationary in the sky, the distance between the remotes to the satellite to the hub is known since the satellite is not moving. Therefore the propagation delay between the remotes to the hub via the satellite is known because the distance between them is fixed. If one remote is closer to the satellite than a second remote, then the first remote will transmit with a delay that is somewhat less than a delay associated with the second remote. The different delays (based on the different geographical locations of the remotes, respectively) ensure that the satellite will receive the transmissions from the first remote and the second remote at the correctly specified TDMA time slots.
If the satellite is moving then the distance between the remotes and the hub is constantly changing. In these situations, inroute (remote to hub) frame synchronization is essential for any TDMA based system to allow multiple remotes to use a single inroute frequency without collisions.
In traditional communication systems, ephemeris data is used to determine the satellite motion, which describes the path that the satellite is following as it orbits the earth. Accordingly, ephemeris data is needed at the gateway.
Another known method called “hub signal loopback” requires hub being capable of receiving its own transmit signal in order to provide hub to satellite round trip delay for tracking the satellite motion. In this case, a remote is co-located at the hub. When the hub transmits a signal, it is echoed back to the remote that is sitting right next to the hub. The hub can then determine the propagation delay by measuring the delay between the transmission and the reception of the signal and therefore controls the timing of the remotes. However, in certain systems, the hub is not located inside a spot beam, and hence is not able to receive what it had transmitted earlier. In this situation the “hub signal loopback” is not applicable.
What is needed is a system and method to provide inroute frame synchronization for tracking and compensating of the satellite motion to allow multiple remotes to use TDMA on the inroute frequencies.